Heads, governors and parents feared strikes could break the consensus with teachers over the campaign for better education funding - seen in recent months at demonstrations and lobbies over local government budgets.

David Hart, general secretary of the National Association of Head Teachers, said: “I don’t believe you can march arm in arm with parents in support of a funding campaign one day and then turn round and say you are sending their children home. Tactically it will not help the funding campaign - the Government will make capital out of it.”

He added that if the National Association of Schoolmasters Union of Women Teachers members refused to take large classes, heads would be forced to choose which pupils were excluded on a rota basis to ensure the damage to their education was evenly spread. But Mr Hart denied this was effectively co-operating with those taking the action. “What they would be doing is minimising the loss to the children.”

The National Governors Council also doubts if the action will achieve anything. Chairman Simon Goodenough said a single day’s action would be largely ignored while systematic strikes would anger parents by damaging children’s prospects.

He said: “There is no point taking action if you don’t know what you are setting out to achieve. The whole issue of the priority given to education will not be resolved in a summer of discontent.”

The NASUWT wants to use health and safety legislation to help reduce teacher workload.

Allan Craig, secretary of the Kent federation said children in his area will be sent home more because of unsafe buildings than any industrial action. Overcrowding reduced effectiveness of fire exits, he said.

But apart from craft, design and technology lessons which - because of potentially dangerous equipment in the rooms - must by law have at least one member of staff for every 20 pupils, Parliament gives few weapons to campaigners.

Dick Hill, secretary of the education service advisory committee of the Health and Safety Executive, said classroom accidents in England and Wales fell from 379 three years ago to 268 last year. Although there were occasional complaints to the HSE about crowded classrooms, it had never enforced the removal of pupils.

Any teacher who refused to take a crowded class would have some backing if “there was a real chance that there was so much overcrowding that teachers or pupils were falling over each other,” he said.

Martin Gomberg, education advisor of the Royal Society for the Prevention of Accidents, said any teacher who walked out of a crowded classroom would be stepping into a legal unknown. But he added: “There has to come a time when class size will have an effect on health and safety. The difficulty is where you draw the line.”

In Scotland, teachers’ service conditions include maximum class sizes - 33 for single age primary pupils and the first two secondary years; 30 in the remaining years and 20 in practical subjects.

The NASUWT want a similar system in England and Wales, but Tino Ferri, its Scottish executive members warned it was not a complete solution.

“With modern teaching methodologies like child centred learning, we wouldn’t want our colleagues to think that 33 in a class is a panacea for all they crave,” he said.

Employers are unlikely to want class limits built into contracts.

Andrew Inett from the Local Government Management Board said such a measure would reduce the flexibility schools enjoyed under local management. “If the funding was there without the statutory basis, I am sure schools would choose not to have excessive-sized classes.”